1. Field of the Invention
The present invention relates to a control circuit and method for dimming a lighting device.
2. Background Art
One method for dimming a lighting device or the like is a pulse width modulation (PWM) system that switches on and off a light source such as a light emitting diode (LED) by a pulse signal having a constant frequency and it modulated duty ratio (refer to, for example, U.S. Pat. Nos. 8,294,388, 8,154,222, 8,198,832 and 7,321,203 and Japanese Patent Application Laid-Open No. 2011-9366).
In this PWM system, for example, a direct current-to-direct current (DC-DC) converter, which usually has excellent conversion efficiency, is used as a constant current source and a constant voltage regulator. A duty ratio, which is a ratio of the width of an on state to one period of a drive pulse, i.e., an energizing portion during one period of the drive pulse, is changed to adjust the on/off time, controlling the brightness or luminance of the lighting device.
FIG. 1 is a diagram illustrating a configuration of an example of a PWM dimming control circuit. As illustrated in FIG. 1, is DC-DC convertor 1 is a buck converter, i.e. a converter of step-down type, and functions to cause a constant current to flow through a light source 2, such as an LED, by using a PWM control signal from a constant current controller 4. The constant current controller 4 is connected to a brightness modification unit 3 and a node located at the low side of the light source 2. The constant current controller 4 controls the operation of a switch in the DC-DC converter 1 based on a brightness control signal from the brightness modification unit 3 and a feedback signal indicative of a light emission state of the light source 2.